(1) Field of the Invention
The present invention relates generally to acoustic ranging systems and, more particularly, to an acoustic ranging system for passive receiver determination of range to an underwater object utilizing one or more remotely located acoustic transmitters.
(2) Description of the Prior Art
The determination of range to an underwater object has historically been accomplished by one of two basic methodologies, passive and active.
Passive systems, typified by those in use in submarines for finding underwater adversaries, have known weaknesses. The target object must be radiating acoustic energy into the medium (water), so this method is useless against non-radiating targets and is directly dependent on the amount of acoustic radiation for radiating targets. Examples of completely non-radiating target objects include geographic features, submerged wreckage, and mines. Examples of weakly radiating target objects include diesel-electric submarines bottomed or otherwise operating on battery power.
Even for strongly radiating target objects, the range to the object must often be determined by indirect means involving receiver motion to generate time rates of change of bearing to the object or similar indirect techniques. These techniques are typically time consuming and prone to significant errors based on geometric configurations of the receiver and the target object. Signal strength at the receiver may also be used to estimate range but is notoriously inexact due to the propagation characteristics of sound in water and the necessity to know the precise acoustic energy output of the target object. Target acoustic energy output is not typically known except in controlled experimental conditions.
Active systems are often characterized by a receiver able to discern bearing in at least one plane and an active transmitter either collocated with the receiver or remote from the receiver. Prior art active systems also have known limitations. For instance, in the military application, the use of an active transmitter collocated with the receiver (mounted for this example on a submarine), may provide clues to the presence and location of the submarine. This potentially results in the target being alerted to the fact that it is under prosecution and evading. Active radiation for targeting localization purposes also provides a very good reverse bearing location to the transmitter, making an effective counterattack possible and placing the transmitting ship in jeopardy.
Locating the transmitter remotely from the receiver in certain types of active systems, sometimes referred to as “multi-static” systems, requires precise a priori knowledge of the location of the transmitter relative to the receiver, or the provision of that information simultaneous with the transmission via some type of communications link. For most maritime applications, it is desirable that at least the receiver is mobile, and usually that both transmitter and receiver be mobile, for purposes of portability and mission flexibility.
There are several different types of prior art active ranging systems which use remotely located transmitters. Each of these has disadvantages:
In an active ranging system with a mobile receiver and fixed transmitter there is a need to know with great accuracy the location of the fixed transmitter relative to the receiver platform. This is accomplished by techniques involving either precise geolocation of the transmitter at time of implantation and ongoing precise geolocation of the receiver platform as it moves around, or by techniques requiring ongoing communication between the receiver and the transmitter to pass locating information. While useful in undersea navigation applications, this technique has the weaknesses of non-portability and mission inflexibility (the fixed transmitter is only useful to searching vessel in range of its location), as well as requiring a continuous and reliable communications channel between transmitter and receiver.
For ranging systems that utilize a mobile receiver and mobile transmitter, precise knowledge of both mobile transmitter and mobile receiver geolocations is required. Accordingly, these systems require both sensitive navigational equipment on both platforms and a method of transferring communication information between the two.
Ranging systems that utilize a fixed receiver and a mobile transmitter are not known to have any useful applications, although a potential application in coastal surveillance can be readily envisioned. However, the limitations of this type of system would be the same as discussed above for the ranging system with a mobile receiver and fixed transmitter.
The following U.S. patents describe various prior art systems that may be related to the above and/or other ranging systems:
U.S. Pat. No. 4,173,008, issued Oct. 30, 1979, to Keith et al, discloses a method and apparatus for passively detecting, locating and tracking submerged or water-borne objects which generate acoustic radiation as they move through the water, including a hydrophone array which is linked to a monitoring station by cable or radio transmitters. The apparatus at the monitoring station includes a hydrophone selector for individually monitoring the output of each hydrophone, a band pass filter, a db meter and visual display apparatus. The db meter is provided with indicia to convert the difference in intensity in db for the hydrophones to a range or distance ratio from the hydrophones to the noise source. A fix on the noise source can be obtained by plotting range ratio circles or using a tactical display scope. Once the noise source is located, the db meter is calibrated to read directly the range from the hydrophones to the object.
U.S. Pat. No. 4,499,468, issued Feb. 12, 1985, to Montana et al, discloses a range-only multistatic radar system that has a plurality of radar stations located in a square grid pattern. Each radar station has a plurality of low power pulsed transmitters and receivers that operate on omnidirectional whip antennae. Echo information received by the receivers is transmitted to a computer wherein a technique is employed to deghost and identify real targets. The computer applies a range difference similarity test, a uniqueness test, and a position test to determine real targets from the plurality of echo returns received.
U.S. Pat. No. 5,168,471, issued Dec. 1, 1992, to Parra, discloses an integrated active/passive transducer system that includes an active transducer for converting ultrasonic electric energy to pressure waves launched in a body of water and converting reflected ultrasonic pressure waves from a target in the body of water to a first electrical signal. A passive transducer is coaxial with the active transducer, converting biosound pressure waves from a living organism in the body of water to second electrical signals. A first processing channel is connected to receive the first electrical signals and determine range to the target. A second processing channel is connected to receive the second electrical signals and provide a biosound signal. A logic system is connected to receive the first electrical signal and said biosound signal, to validate that the target is a living organism and not an inanimate object only in the presence of biosound signals, and an indication is connected to the logic means for indicating that the target is a living organism. The logic system includes a microprocessor and a temperature sensor measures the temperature of the body of water contiguous to the transducers and produces an electrical signal corresponding thereto. The microprocessor is programmed to correlate the biosound signals with the thermotropic characteristics, etc., of the living organism.
U.S. Pat. No. 5,168,473, issued Dec. 1, 1992, to Parra, discloses one or more passive transducers for converting sounds, including bio-soundwaves from a living aquatic animal source traveling in a body of water, to electrical signals, which is combined with active sonar (echo sounding) to form an integrated marine acoustic system. Moreover, a common display can be used and coupled with a satellite (GPS) or loran position locator so that a fishing vessel carrying the apparatus can have its position displayed on an electronically reproduced chart or map of an area along with the fish location, including an indication of the direction of fish movement, and fish species.
The above cited prior art does not disclose a ranging system that overcomes the problems discussed above. Accordingly, the solutions to the above described and/or related problems have been long sought without success. Consequently, those skilled in the art will appreciate the present invention that addresses the above and other problems.